The disclosure is based on a ring element to be arranged at at least one axial end region of a winding section of an electric motor rotor. Furthermore, the disclosure relates to a lamellar insulating element which comprises a ring element according to the disclosure. Finally, the disclosure relates to an electric motor on the rotor of which such a ring element is arranged, and to an electrical power tool that comprises an electric motor according to the disclosure.
Universal motors, i.e. series-wound motors in which the rotors bear a rotor winding that is supplied with current via a commutator, are generally used in electric motors for electric power tools, in particular an angle grinder, a drill, a saw, jigsaws or the like. In the region of the end windings at the axial end regions of a winding section, the inner layers of the winding in particular have a small radius of curvature and are exposed to high mechanical and thermal loads. For instance, in the compact installation space, the inner layers of the rotor windings have a very small bending radius, the wire coils being exposed to a high thermal loads and having to withstand centrifugal forces. It is known in this respect to use fixing of the winding layers by means of a trickle impregnating resin for improved connection, with the aim of obtaining a homogeneous winding cross section and maintaining minimum bending radii. However, the innermost winding layer cannot be definitively fixed in the winding assembly, lying as it were in free space in the region of the end windings at one axial end region of the winding section of the electric motor rotor and resin fixing being unable to take hold.
This gives rise to the problem that the inner winding layer is not sufficiently fixed in the region of the end windings and is insufficiently protected from both thermal and mechanical loads, in particular from high vibrations and centrifugal forces.
In particular under dynamic load changes, such as take place for example in the case of electric power tools operating at full load and thereafter when idling at high speeds, the temperature of the rotor windings rises to above 210° C., and so a critical threshold temperature for the enamel insulation is reached. It is thereby found that, in 70% of these cases of failure, the inter-winding shorts are in turn caused in the inner layers of the end windings, and so approximately 50% of all cases of motor failure are attributable to inter-winding shorts in the innermost layers of the end windings.